A pressure-control valve having prepressure compensation is disclosed in published German patent application DE-OS 26 28 371. The valve is disposed between a primary pressure chamber and a secondary pressure chamber and includes a valve body lying in contact engagement with a valve seat. The valve is connected to a movable pin which functions as a compensating piston. The prepressure compensation takes place in such a manner that two channels branch from the supply connection for the primary pressure. On the one hand, the channels apply pressure to the valve body and on the other hand, to the compensating piston. The primary pressure acting via the first channel on the compensating piston presses the valve body against the valve seat while the primary pressure present in the second channel generates an oppositely directed force on the valve body. Complete compensation is then present when the cross-sectional surface of the compensating piston charged with the primary pressure is equal to the clear cross-sectional surface of the valve seat so that, in this way, there is a force balance between the resulting primary pressure forces acting in the axial direction. The compensating piston is guided in a cylindrical bore and is sealed with an O-ring as a sealing unit.
Swiss patent 466,051 discloses a lung-controlled prepressure compensated compressed air ventilating apparatus that includes a valve body and a compensating piston which are rigidly connected to each other. The primary pressure acts on the valve body as well as on the compensating piston so that the resulting axial forces are in balance. The compensating piston is guided in a cylindrical bore of a compensating housing disposed between the primary pressure chamber and the secondary pressure chamber and is seated in a gas-tight manner with a sealing unit in the form of an O-ring seal. To open the valve, a tilt lever connected to the control membrane presses against the compensating piston with the valve body moving into the open position. The pressure gas then flows from the primary pressure chamber into the secondary pressure chamber.
With these apparatus, it is a disadvantage that frictional forces have to be overcome for displacing the compensating piston with the friction forces being caused essentially by the O-ring seal on the compensating piston. The friction force is furthermore influenced by the primary pressure acting on the sealing ring. Because of the so-called slip-stick effect on the sealing unit, pressure fluctuations occur on the secondary pressure side for static as well as dynamic loads. The pressure fluctuations make the operation of metering apparatus and especially of breathing-controlled metering apparatus difficult.
Published and examined German patent application DE-AS 11 48 040 discloses a pressure-reducing valve having a pressure-compensated compensating piston which is seated in the compensating housing with a sealing arrangement in the form of a folded-over seal. A disadvantage of this arrangement is that for securing and sealing the folded-over seal in the compensating housing, a valve disc is required which presses the attachment bead of the seal into a cut-away portion of the cylindrical wall of the compensating housing. The assembly of the seal is made difficult by the introduction of the valve disc.